You're guess is as good as mine about the flat. I just figured that since flat 593 was the last active freight car, that it would be the car present in the 1941 roster (and in the 1945 too if I remember right.) Quite frankly, it is a mute point because I highly doubt that any freight car, barring the ex C&S stuff, would have been serviceable by that time.
What I am trying to say about the C&S cars is that, yes, they were used in interchange service. I've seen them in one of my videos over Cerro Summit. But, I don't think that the RGS ever owned the Miller cars. Miller got them as part of a settlement with the C&S, and as far as I can remember, they were never transferred to RGS ownership. So, even though the RGS was just as good as an owner, the cars could probably not be recorded as being owned by the RGS.
Ah, the wonderful simplicities of narrow gauge. Oh brother.